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rfc:rfc7131

Internet Engineering Task Force (IETF) M. Barnes Request for Comments: 7131 Category: Informational F. Audet ISSN: 2070-1721 Skype

                                                           S. Schubert
                                                                   NTT
                                                         H. van Elburg
                                            Detecon International Gmbh
                                                           C. Holmberg
                                                              Ericsson
                                                            March 2014

Session Initiation Protocol (SIP) History-Info Header Call Flow Examples

Abstract

 This document describes use cases and documents call flows that
 require the History-Info header field to capture the Request-URIs as
 a Session Initiation Protocol (SIP) Request is retargeted.  The use
 cases are described along with the corresponding call flow diagrams
 and messaging details.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Not all documents
 approved by the IESG are a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7131.

Barnes, et al. Informational [Page 1] RFC 7131 History-Info Call Flows March 2014

Copyright Notice

 Copyright (c) 2014 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1. Overview ........................................................2
 2. Conventions and Terminology .....................................3
 3. Detailed Call Flows .............................................3
    3.1. Sequentially Forking (History-Info in Response) ............3
    3.2. History-Info with Privacy Header Field ....................11
    3.3. Privacy for a Specific History-Info Entry .................16
    3.4. Automatic Call Distribution ...............................20
    3.5. Determining the Alias Used ................................27
    3.6. PBX Voicemail Example .....................................29
    3.7. Consumer Voicemail Example ................................35
    3.8. GRUU ......................................................41
    3.9. Limited-Use Address .......................................44
    3.10. Service Invocation .......................................47
    3.11. Toll-Free Number .........................................48
 4. Security Considerations ........................................51
 5. Acknowledgements ...............................................51
 6. Informative References .........................................51

1. Overview

 Many services that use SIP require the ability to determine why and
 how the call arrived at a specific application.  The use cases
 provided in this document illustrate the use of the History-Info
 header [RFC7044], for example, applications and common scenarios.
 The optional "rc" and "mp" header field parameters defined in
 [RFC7044] are required for several of the use cases.  Descriptions of
 the example use cases, call flow diagrams, and messaging details are
 provided.

Barnes, et al. Informational [Page 2] RFC 7131 History-Info Call Flows March 2014

2. Conventions and Terminology

 The term "retarget" is used as defined in [RFC7044].  The terms
 "location service", "redirect", and "address-of-record (AOR)" are
 used consistent with the terminology in [RFC3261].

3. Detailed Call Flows

 The scenarios in this section provide sample use cases for the
 History-Info header for informational purposes only.  They are not
 intended to be normative.  In many cases, only the relevant messaging
 details are included in the body of the call flow.

3.1. Sequentially Forking (History-Info in Response)

 This scenario highlights an example where the History-Info in the
 response is useful to an application or user that originated the
 request.
 Alice sends a call to Bob via sip:example.com.  The proxy
 sip:example.com sequentially tries Bob on a SIP User Agent (UA) that
 has bound a contact with the sip:bob@example.com AOR, and then
 several alternate addresses (Office and Home) unsuccessfully before
 sending a response to Alice.  The hi-entry containing the initial
 contact is the hi-entry just prior to the first hi-entry tagged with
 an "rc" header field parameter.  In this example, the Office and Home
 are not the same AOR as sip:bob@example.com, but rather different
 AORs that have been configured as alternate addresses for Bob in the
 proxy.  In other words, Office and Home are not bound through SIP
 Registration with Bob's AOR.  This type of arrangement is common, for
 example, when a "routing" rule to a Public Switched Telephone Network
 (PSTN) number is manually configured in a proxy.  These hi-entries
 are identified by the index contained in the hi-target-param "mp"
 header field parameter in the hi-entries.
 This scenario illustrates that by providing the History-Info to
 Alice, the end-user, or an application at Alice could make a decision
 on how best to attempt finding Bob without sending multiple requests
 to the same destination.  Upon receipt of the response containing the
 History-Info entries, the Request-URIs for the History-Info entries
 tagged with an "mp" header field parameter are extracted.  Those
 Request-URIs can be compared to other URIs (if any) that might be
 attempted in order to establish the session with Bob.  This results
 in avoiding the sending of another INVITE to Bob's home phone.
 Without this mechanism, Alice might well attempt to reach Bob at his
 office phone, which would then retarget the request to Bob's home
 phone.  When that attempt failed, then Alice might attempt to reach
 Bob directly at his home phone, unknowingly for a third time.

Barnes, et al. Informational [Page 3] RFC 7131 History-Info Call Flows March 2014

   Alice   example.com            Bob     Office    Home
   |            |                  |        |        |
   | INVITE F1  |                  |        |        |
   |----------->|    INVITE F2     |        |        |
   |            |----------------->|        |        |
   | 100 Trying F3                 |        |        |
   |<-----------|  302 Move Temporarily F4  |        |
   |            |<-----------------|        |        |
   |            |   ACK F5         |        |        |
   |            |----------------->|        |        |
   |            |       INVITE F6           |        |
   |            |-------------------------->|        |
   |            |      180 Ringing F7       |        |
   |            |<--------------------------|        |
   |  180 Ringing F8                        |        |
   |<-----------|   retransmit INVITE       |        |
   |            |-------------------------->|        |
   |            |      ( timeout )          |        |
   |            |             INVITE F9              |
   |            |----------------------------------->|
   |            |           100 Trying F10           |
   |            |<-----------------------------------|
   |            |           486 Busy Here F11        |
   |            |<-----------------------------------|
   |  486 Busy Here F12                              |
   |<-----------|             ACK F13                |
   |            |----------------------------------->|
   |  ACK F14   |                                    |
   |----------->|                                    |
               Figure 1: Example with Sequential Forking

Barnes, et al. Informational [Page 4] RFC 7131 History-Info Call Flows March 2014

 Message Details
 F1 INVITE Alice -> example.com
 INVITE sip:bob@example.com SIP/2.0
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK4321
 Max-Forwards: 70
 From: Alice <sip:alice@example.com>;tag=sr3dds
 To: Bob <sip:bob@example.com>
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 History-Info: <sip:bob@example.com>;index=1
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]
 F2 INVITE  example.com -> Bob
 INVITE sip:bob@192.0.2.4 SIP/2.0
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bKx3st
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK4321
 Max-Forwards: 69
 From: Alice <sip:alice@example.com>;tag=sr3dds
 To: Bob <sip:bob@example.com>
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 Record-Route: <sip:proxy.example.com;lr>
 History-Info: <sip:bob@example.com>;index=1
 History-Info: <sip:bob@192.0.2.4>;index=1.1;rc=1
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]

Barnes, et al. Informational [Page 5] RFC 7131 History-Info Call Flows March 2014

 F3 100 Trying example.com -> Alice
 SIP/2.0 100 Trying
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK4321
 From: Alice <sip:alice@example.com>;tag=sr3dds
 To: Bob <sip:bob@example.com>
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 Content-Length: 0
 F4 302 Moved Temporarily Bob -> example.com
 SIP/2.0 302 Moved Temporarily
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bKx3st
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK4321
 From: Alice <sip:alice@example.com>;tag=sr3dds
 To: Bob <sip:bob@example.com>;tag=es43sd
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 History-Info: <sip:bob@example.com>;index=1
 History-Info: <sip:bob@192.0.2.4>;index=1.1;rc=1
 Contact: <sip:office@example.com>;mp=1
 Content-Length: 0
 F5 ACK example.com -> Bob
 ACK sip:bob@example.com SIP/2.0
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bKx3st
 Max-Forwards: 70
 From: Alice <sip:alice@example.com>;tag=sr3dds
 To: Bob <sip:bob@example.com>;tag=es43sd
 Call-ID: 12345600@example.com
 CSeq: 1 ACK
 Content-Length: 0

Barnes, et al. Informational [Page 6] RFC 7131 History-Info Call Flows March 2014

 F6 INVITE example.com -> office
 INVITE sip:office@192.0.2.5 SIP/2.0
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bKx4st
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK4321
 Max-Forwards: 69
 From: Alice <sip:alice@example.com>;tag=sr3dds
 To: Bob <sip:bob@example.com>
 Supported: histinfo
 Call-ID: 12345600@example.com
 Record-Route: <sip:proxy.example.com;lr>
 History-Info: <sip:bob@example.com>;index=1
 History-Info: <sip:bob@192.0.2.4?Reason=SIP%3Bcause%3D302>;\
               index=1.1;rc=1
 History-Info: <sip:office@example.com>;index=1.2;mp=1
 History-Info: <sip:office@192.0.2.5>;index=1.2.1;rc=1.2
 CSeq: 1 INVITE
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]
 F7 180 Ringing office -> example.com
 SIP/2.0 180 Ringing
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bKx4st
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK4321
 From: Alice <sip:alice@example.com>;tag=sr3dds
 To: Bob <sip:bob@example.com>;tag=53rdds
 Supported: histinfo
 Call-ID: 12345600@example.com
 Record-Route: <sip:proxy.example.com;lr>
 History-Info: <sip:bob@example.com>;index=1
 History-Info: <sip:bob@192.0.2.4?Reason=SIP%3Bcause%3D302>;\
               index=1.1;rc=1
 History-Info: <sip:office@example.com>;index=1.2;mp=1
 History-Info: <sip:office@192.0.2.5>;index=1.2.1;rc=1.2
 CSeq: 1 INVITE
 Contact: Office <sip:office@192.0.2.5>
 Content-Length: 0

Barnes, et al. Informational [Page 7] RFC 7131 History-Info Call Flows March 2014

 F8 180 Ringing example.com -> Alice
 SIP/2.0 180 Ringing
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK4321
 From: Alice <sip:alice@example.com>;tag=sr3dds
 To: Bob <sip:bob@example.com>;tag=53rdds
 Supported: histinfo
 Call-ID: 12345600@example.com
 Record-Route: <sip:proxy.example.com;lr>
 History-Info: <sip:bob@example.com>;index=1
 History-Info: <sip:bob@192.0.2.4?Reason=SIP%3Bcause%3D302>;\
               index=1.1;rc=1
 History-Info: <sip:office@example.com>;index=1.2;mp=1
 History-Info: <sip:office@192.0.2.5>;index=1.2.1;rc=1.2
 CSeq: 1 INVITE
 Contact: Office <sip:office@192.0.2.5>
 Content-Length: 0
 F9 INVITE example.com -> home
 INVITE sip:home@192.0.2.6 SIP/2.0
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bKx5st
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK4321
 Max-Forwards: 69
 From: Alice <sip:alice@example.com>;tag=sr3dds
 To: Bob <sip:bob@example.com>
 Supported: histinfo
 Call-ID: 12345600@example.com
 Record-Route: <sip:proxy.example.com;lr>
 History-Info: <sip:bob@example.com>;index=1
 History-Info: <sip:bob@192.0.2.4?Reason=SIP%3Bcause%3D302>;\
               index=1.1;rc=1
 History-Info: <sip:office@example.com?Reason=SIP%3Bcause%3D408>;\
               index=1.2;mp=1
 History-Info: <sip:office@192.0.2.5?Reason=SIP%3Bcause%3D408>;\
               index=1.2.1;rc=1.2
 History-Info: <sip:home@example.com>;index=1.3;mp=1
 History-Info: <sip:home@192.0.2.6>;index=1.3.1;rc=1.3
 CSeq: 1 INVITE
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]

Barnes, et al. Informational [Page 8] RFC 7131 History-Info Call Flows March 2014

 F10 100 Trying home -> example.com
 SIP/2.0 100 Trying
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bKx5st
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK4321
 From: Alice <sip:alice@example.com>;tag=sr3dds
 To: Bob <sip:bob@example.com>
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 Content-Length: 0
 F11 486 Busy Here home -> example.com
 SIP/2.0  486 Busy Here
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bKx5st
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK4321
 From: Alice <sip:alice@example.com>;tag=sr3dds
 To: Bob <sip:bob@example.com>;tag=55rdds
 Call-ID: 12345600@example.com
 History-Info: <sip:bob@example.com>;index=1
 History-Info: <sip:bob@192.0.2.4?Reason=SIP%3Bcause%3D302>;\
               index=1.1;rc=1
 History-Info: <sip:office@example.com?Reason=SIP%3Bcause%3D408>;\
               index=1.2;mp=1
 History-Info: <sip:office@192.0.2.5?Reason=SIP%3Bcause%3D408>;\
               index=1.2.1;rc=1.2
 History-Info: <sip:home@example.com>;index=1.3;mp=1
 History-Info: <sip:home@192.0.2.6>;index=1.3.1;rc=1.3
 CSeq: 1 INVITE
 Content-Length: 0

Barnes, et al. Informational [Page 9] RFC 7131 History-Info Call Flows March 2014

 F12 486 Busy Here example.com -> Alice
 SIP/2.0  486 Busy Here
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK4321
 From: Alice <sip:alice@example.com>;tag=sr3dds
 To: Bob <sip:bob@example.com>;tag=55rdds
 Call-ID: 12345600@example.com
 History-Info: <sip:bob@example.com>;index=1
 History-Info: <sip:bob@192.0.2.4?Reason=SIP%3Bcause%3D302>;\
               index=1.1;rc=1
 History-Info: <sip:office@example.com?Reason=SIP%3Bcause%3D408>;\
               index=1.2;mp=1
 History-Info: <sip:office@192.0.2.5?Reason=SIP%3Bcause%3D408>;\
               index=1.2.1;rc=1.2
 History-Info: <sip:home@example.com>;index=1.3;mp=1
 History-Info: <sip:home@192.0.2.6>;index=1.3.1;rc=1.3
 CSeq: 1 INVITE
 Content-Length: 0
 F13 ACK example.com -> home
 ACK sip:home@192.0.2.6 SIP/2.0
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bKx5st
 Max-Forwards: 70
 From: Alice <sip:alice@example.com>;tag=sr3dds
 To: Bob <sip:bob@example.com>;tag=55rdds
 Call-ID: 12345600@example.com
 CSeq: 1 ACK
 Content-Length: 0
 F14 ACK Alice -> example.com
 ACK sip:bob@example.com SIP/2.0
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK4321
 Max-Forwards: 70
 From: Alice <sip:alice@example.com>;tag=sr3dds
 To: Bob <sip:bob@example.com>;tag=55rdds
 Call-ID: 12345600@example.com
 Route: <sip:proxy.example.com;lr>
 CSeq: 1 ACK
 Content-Length: 0

Barnes, et al. Informational [Page 10] RFC 7131 History-Info Call Flows March 2014

3.2. History-Info with Privacy Header Field

 This is an example of the use of the Privacy header field with a
 value of "history" added by an intermediary.  The intermediary
 responsible for the biloxi.example.com domain adds a Privacy header
 field with a value of "history" indicating that all the History-Info
 header field information is anonymized outside the biloxi.example.com
 domain.
    Alice  atlanta.example.com  biloxi.example.com  Bob Work  Bob Home
    |             |                |                 |          |
    | INVITE F1   |                |                 |          |
    |------------>|                |                 |          |
    |             |                |                 |          |
    |             |   INVITE F2    |                 |          |
    |             |--------------->|                 |          |
    |             |                |                 |          |
    |             |                | INVITE F3       |          |
    |             |                |---------------->|          |
    |             |                |302 Move Temporarily F4     |
    |             |                |<----------------|          |
    |             |                |    ACK F5       |          |
    |             |                |---------------->|          |
    |             |                |                 |          |
    |             |                | INVITE F6       |          |
    |             |                |--------------------------->|
    |             |                |     200 F7      |          |
    |             |                |<---------------------------|
    |             |                |                 |          |
    |             |     200 F8     |                 |          |
    |             |<---------------|                 |          |
    |             |                |                 |          |
    |     200 F9  |                |                 |          |
    |<------------|                |                 |          |
    |             |                |                 |          |
    |             |       ACK      |                 |          |
    |---------------------------------------------------------->|
    |             |                |                 |          |
             Figure 2: Example with Privacy Header Fields

Barnes, et al. Informational [Page 11] RFC 7131 History-Info Call Flows March 2014

 Message Details
 F1 INVITE Alice -> atlanta.example.com
 INVITE sip:bob@biloxi.example.com;p=x SIP/2.0
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK4321
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=22
 To: Bob <sip:bob@biloxi.example.com>
 Supported: histinfo
 Privacy: history
 Call-ID: 12345600@atlanta.example.com
 CSeq: 1 INVITE
 History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]
 F2 INVITE  atlanta.example.com -> biloxi.example.com
 INVITE sip:bob@biloxi.example.com;p=x SIP/2.0
 Via: SIP/2.0/TCP proxy.atlanta.example.com:5060;branch=z9hG4bKbst2
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK4321
 Max-Forwards: 69
 From: Alice <sip:alice@atlanta.example.com>;tag=22
 To: Bob <sip:bob@biloxi.example.com>
 Supported: histinfo
 Call-ID: 12345600@atlanta.example.com
 CSeq: 1 INVITE
 History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
 History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]

Barnes, et al. Informational [Page 12] RFC 7131 History-Info Call Flows March 2014

 F3 INVITE  biloxi.example.com -> Bob Work
 INVITE sip:bob@192.0.1.11 SIP/2.0
 Via: SIP/2.0/TCP proxy.biloxi.example.com:5060;branch=z9hG4bKgs33
 Via: SIP/2.0/TCP proxy.atlanta.example.com:5060;branch=z9hG4bKbst2;\
                received=192.0.2.3
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK4321
 Max-Forwards: 68
 From: Alice <sip:alice@atlanta.example.com>;tag=22
 To: Bob <sip:bob@biloxi.example.com>
 Privacy: history
 Supported: histinfo
 Call-ID: 12345600@atlanta.example.com
 CSeq: 1 INVITE
 History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
 History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1
 History-Info: <sip:bob@192.0.1.11>;index=1.1.1;rc=1.1
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]
 F4 302 Moved Temporarily Bob Work -> biloxi.example.com
 SIP/2.0 302 Moved Temporarily
 Via: SIP/2.0/TCP proxy.biloxi.example.com:5060;branch=z9hG4bKgs33;\
                  received=192.0.2.102
 Via: SIP/2.0/TCP proxy.atlanta.example.com:5060;branch=z9hG4bKbst2;\
                received=192.0.2.3
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK4321
 From: Alice <sip:alice@atlanta.example.com>;tag=22
 To: Bob <sip:bob@biloxi.example.com>;tag=11
 Privacy: history
 Supported: histinfo
 Call-ID: 12345600@atlanta.example.com
 CSeq: 1 INVITE
 History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
 History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1
 History-Info: <sip:bob@192.0.1.11>;index=1.1.1;rc=1.1
 Contact: Bob Home <sip:bob@192.0.1.15>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]

Barnes, et al. Informational [Page 13] RFC 7131 History-Info Call Flows March 2014

 F5 ACK  biloxi.example.com -> Bob Work
 ACK sip:bob@192.0.1.11 SIP/2.0
 Via: SIP/2.0/TCP proxy.biloxi.example.com:5060;branch=z9hG4bKgs33
 Via: SIP/2.0/TCP proxy.atlanta.example.com:5060;branch=z9hG4bKbst2;\
                received=192.0.2.3
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK4321
 Max-Forwards: 68
 From: Alice <sip:alice@atlanta.example.com>;tag=22
 To: Bob <sip:bob@biloxi.example.com>;tag=11
 Call-ID: 12345600@atlanta.example.com
 CSeq: 1 ACK
 Content-Length: <appropriate value>
 [SDP Not Shown]
 F6 INVITE  biloxi.example.com -> Bob Home
 INVITE sip:bob@192.0.1.15 SIP/2.0
 Via: SIP/2.0/TCP proxy.biloxi.example.com:5060;branch=z9hG4bKgs32
 Via: SIP/2.0/TCP proxy.atlanta.example.com:5060;branch=z9hG4bKbst2;\
                received=192.0.2.3
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK4321
 Max-Forwards: 68
 From: Alice <sip:alice@atlanta.example.com>;tag=22
 To: Bob <sip:bob@biloxi.example.com>
 Privacy: history
 Supported: histinfo
 Call-ID: 12345600@atlanta.example.com
 CSeq: 1 INVITE
 History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
 History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1
 History-Info: <sip:bob@192.0.1.11?Reason=SIP%3Bcause%3D302>;\
                 index=1.1.1;rc=1
 History-Info: <sip:bob@192.0.1.15>;index=1.1.2
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]

Barnes, et al. Informational [Page 14] RFC 7131 History-Info Call Flows March 2014

 F7 200 OK  Bob -> biloxi.example.com
 SIP/2.0 200 OK
 Via: SIP/2.0/TCP proxy.biloxi.example.com:5060;branch=z9hG4bKgs32;\
                received=192.0.2.101
 Via: SIP/2.0/TCP proxy.atlanta.example.com:5060;branch=z9hG4bKbst2;\
                received=192.0.2.3
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK4321
 From: Alice <sip:alice@atlanta.example.com>;tag=22
 To: Bob <sip:bob@biloxi.example.com>;tag=33
 Privacy: history
 Supported: histinfo
 Call-ID: 12345600@atlanta.example.com
 CSeq: 1 INVITE
 History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
 History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1
 History-Info: <sip:bob@192.0.1.11?Reason=SIP%3Bcause%3D302>;\
                 index=1.1.1;rc=1
 History-Info: <sip:bob@192.0.1.15>;index=1.1.2;rc=1.1
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]
 F8 200 OK  biloxi.example.com -> atlanta.example.com
 SIP/2.0 200 OK
 Via: SIP/2.0/TCP proxy.atlanta.example.com:5060;branch=z9hG4bKbst2;\
                received=192.0.2.3
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK4321
 From: Alice <sip:alice@atlanta.example.com>;tag=22
 To: Bob <sip:bob@biloxi.example.com>;tag=33
 Privacy: history
 Supported: histinfo
 Call-ID: 12345600@atlanta.example.com
 CSeq: 1 INVITE
 History-Info: <sip:anonymous@anonymous.invalid>;index=1
 History-Info: <sip:anonymous@anonymous.invalid>;index=1.1
 History-Info: <sip:anonymous@anonymous.invalid>;index=1.1.1;rc=1
 History-Info: <sip:anonymous@anonymous.invalid>;index=1.1.2;rc=1.1
 Contact: Bob <sip:bob@192.0.1.11>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]

Barnes, et al. Informational [Page 15] RFC 7131 History-Info Call Flows March 2014

 F9 200 OK  atlanta.example.com -> Alice
 SIP/2.0 200 OK
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK4321
 From: Alice <sip:alice@atlanta.example.com>;tag=22
 To: Bob <sip:bob@biloxi.example.com>;tag=33
 Privacy: history
 Supported: histinfo
 Call-ID: 12345600@atlanta.example.com
 CSeq: 1 INVITE
 History-Info: <sip:anonymous@anonymous.invalid>;index=1
 History-Info: <sip:anonymous@anonymous.invalid>;index=1.1
 History-Info: <sip:anonymous@anonymous.invalid>;index=1.1.1;rc=1
 History-Info: <sip:anonymous@anonymous.invalid>;index=1.1.2;rc=1.1
 Contact: Bob <sip:bob@192.0.1.11>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]

3.3. Privacy for a Specific History-Info Entry

 This example provides a basic call scenario similar to Section 3.2;
 however, due to local policy at sip:biloxi.example.com, only the
 final hi-entry in the History-Info, which is Bob's local URI,
 contains a privacy header field with a priv-value of "history", thus
 providing Alice with some information about the history of the
 request, but anonymizing Bob's local URI.

Barnes, et al. Informational [Page 16] RFC 7131 History-Info Call Flows March 2014

 Alice   atlanta.example.com  biloxi.example.com   Bob
 |                |                |                |
 |   INVITE F1    |                |                |
 |--------------->|                |                |
 |                |                |                |
 |                |   INVITE F2    |                |
 |                |--------------->|                |
 |                |                |                |
 |                |                | INVITE F3      |
 |                |                |--------------->|
 |                |                |                |
 |                |                |     200 F4     |
 |                |                |<---------------|
 |                |                |                |
 |                |     200 F5     |                |
 |                |<---------------|                |
 |                |                |                |
 |     200 F6     |                |                |
 |<---------------|                |                |
 |                |                |                |
 |                |       ACK      |                |
 |------------------------------------------------->|
 |                |                |                |
     Figure 3: Example with Privacy Header Field for Specific URI
 Message Details
 F1 INVITE Alice -> atlanta.example.com
 INVITE sip:bob@biloxi.example.com;p=x SIP/2.0
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK4321
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=22
 To: Bob <sip:bob@biloxi.example.com>
 Supported: histinfo
 Call-ID: 12345600@atlanta.example.com
 CSeq: 1 INVITE
 History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]

Barnes, et al. Informational [Page 17] RFC 7131 History-Info Call Flows March 2014

 F2 INVITE  atlanta.example.com -> biloxi.example.com
 INVITE sip:bob@biloxi.example.com;p=x SIP/2.0
 Via: SIP/2.0/TCP proxy.atlanta.example.com:5060;branch=z9hG4bKbst2
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK4321
 Max-Forwards: 69
 From: Alice <sip:alice@atlanta.example.com>;tag=22
 To: Bob <sip:bob@biloxi.example.com>
 Supported: histinfo
 Call-ID: 12345600@atlanta.example.com
 CSeq: 1 INVITE
 History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
 History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1;np=1
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]
 F3 INVITE  biloxi.example.com -> Bob
 INVITE sip:bob@192.0.1.11 SIP/2.0
 Via: SIP/2.0/TCP proxy.biloxi.example.com:5060;branch=z9hG4bKeset
 Via: SIP/2.0/TCP proxy.atlanta.example.com:5060;branch=z9hG4bKbst2;\
                received=192.0.2.101
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK4321
 Max-Forwards: 68
 From: Alice <sip:alice@atlanta.example.com>;tag=22
 To: Bob <sip:bob@biloxi.example.com>
 Supported: histinfo
 Call-ID: 12345600@atlanta.example.com
 CSeq: 1 INVITE
 History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
 History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1;np=1
 History-Info: <sip:bob@192.0.1.11?Privacy=history>;index=1.1.1;rc=1.1
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]

Barnes, et al. Informational [Page 18] RFC 7131 History-Info Call Flows March 2014

 F4 200 OK  Bob -> biloxi.example.com
 SIP/2.0 200 OK
 Via: SIP/2.0/TCP proxy.biloxi.example.com:5060;branch=z9hG4bKeset;\
                received=192.0.2.5
 Via: SIP/2.0/TCP proxy.atlanta.example.com:5060;branch=z9hG4bKbst2;\
                received=192.0.2.101
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK4321
 From: Alice <sip:alice@atlanta.example.com>;tag=22
 To: Bob <sip:bob@biloxi.example.com>;tag=33
 Supported: histinfo
 Call-ID: 12345600@atlanta.example.com
 CSeq: 1 INVITE
 History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
 History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1;np=1
 History-Info: <sip:bob@192.0.1.11?Privacy=history>;index=1.1.1;rc=1.1
 Contact: Bob <sip:bob@192.0.1.11>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]
 F5 200 OK  biloxi.example.com -> atlanta.example.com
 SIP/2.0 200 OK
 Via: SIP/2.0/TCP proxy.atlanta.example.com:5060;branch=z9hG4bKbst2;\
                received=192.0.2.101
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK4321
 From: Alice <sip:alice@atlanta.example.com>;tag=22
 To: Bob <sip:bob@biloxi.example.com>;tag=33
 Supported: histinfo
 Call-ID: 12345600@atlanta.example.com
 CSeq: 1 INVITE
 History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
 History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1;np=1
 History-Info: <sip:anonymous@anonymous.invalid>;index=1.1.1;rc=1.1
 Contact: Bob <sip:bob@192.0.1.11>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]

Barnes, et al. Informational [Page 19] RFC 7131 History-Info Call Flows March 2014

 F6 200 OK  atlanta.example.com -> Alice
 SIP/2.0 200 OK
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK4321
 From: Alice <sip:alice@atlanta.example.com>;tag=22
 To: Bob <sip:bob@biloxi.example.com>;tag=33
 Supported: histinfo
 Call-ID: 12345600@atlanta.example.com
 CSeq: 1 INVITE
 History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
 History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1;np=1
 History-Info: <sip:anonymous@anonymous.invalid>;index=1.1.1;rc=1.1
 Contact: Bob <sip:bob@192.0.1.11>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]

3.4. Automatic Call Distribution

 This scenario highlights an example of an Automatic Call Distribution
 service, where the agents are divided into groups based upon the type
 of customers they handle.  In this example, the Gold customers are
 given higher priority than Silver customers, so a Gold call would get
 serviced even if all the agents servicing the Gold group were busy,
 by retargeting the request to the Silver Group for delivery to an
 agent.  Upon receipt of the call at the agent assigned to handle the
 incoming call, based upon the History-Info header in the message, the
 application at the agent can provide an indication that this is a
 Gold call by extracting the hi-entry associated with the incoming
 request, which is determined by locating the hi-entry whose index is
 reflected in the first hi-entry with a hi-target of "mp".  In the
 example, this would be the hi-entry referenced by the value of the
 first "mp" header field parameter, i.e., the hi-entry containing an
 index of "1".  An application can also determine how many groups from
 which the call may have overflowed before reaching the agent, etc.,
 and present the information to the agent so that the call can be
 handled appropriately, i.e., "I'm so sorry for the delay, blah, blah,
 blah..."
 For scenarios whereby calls might overflow from the Silver to the
 Gold, clearly the alternate group identification, internal routing,
 or actual agent that handles the call should not be sent to UA1.
 Thus, for this scenario, one would expect that the proxy would not
 support the sending of the History-Info in the response, even if
 requested by Alice or the proxy could anonymize the Silver related
 hi-entries by adding privacy in the Silver hi-entries.

Barnes, et al. Informational [Page 20] RFC 7131 History-Info Call Flows March 2014

 As with the other examples, this is not a complete prescription of
 how one would do this type of service but an example of a subset of
 processing that might be associated with such a service.  In
 addition, this example does not address any aspects of agent
 availability resulting in the call being sent to an agent in another
 group, which might also be done via a SIP interface.
 Alice       example.com     Gold          Silver       Agent
 |              |              |             |            |
 | INVITE F1    |              |             |            |
 |------------->|              |             |            |
 |              |              |             |            |
 |              |  INVITE F2   |             |            |
 |              |------------->|             |            |
 |              |              |             |            |
 |              |  302 Moved Temporarily F3  |            |
 |              |<-------------|             |            |
 |              |              |             |            |
 |              |      ACK     |             |            |
 |              |------------->|             |            |
 |              |              |             |            |
 |              |  INVITE F4   |             |            |
 |              |--------------------------->|            |
 |              |              |             |            |
 |              |              |             | INVITE F5  |
 |              |              |             |----------->|
 |              |              |             |            |
 |              |              |             |  200 OK F6 |
 |              |              |             |<-----------|
 |              |              |             |            |
 |              |         200 OK F7          |            |
 |              |<---------------------------|            |
 |              |              |             |            |
 |  200 OK F8   |              |             |            |
 |<-------------|              |             |            |
 |              |              |             |            |
 |                         ACK F9                         |
 |------------------------------------------------------->|
         Figure 4: Example for Automatic Call Distribution

Barnes, et al. Informational [Page 21] RFC 7131 History-Info Call Flows March 2014

 Message Details
 F1 INVITE Alice -> example.com
 INVITE sip:Gold@example.com SIP/2.0
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK42t2
 Max-Forwards: 70
 From: Alice <sip:alice@example.com>;tag=1235
 To: Gold Member Assistance <sip:Gold@example.com>
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 History-Info: <sip:Gold@example.com>;index=1
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]
 F2 INVITE example.com -> Gold.example.com
 INVITE sip:Gold@gold.example.com SIP/2.0
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bK12s4
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK42t2
 Max-Forwards: 69
 From: Alice <sip:alice@example.com>;tag=1235
 To: Gold Member Assistance <sip:Gold@example.com>
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 History-Info: <sip:Gold@example.com>;index=1
 History-Info: <sip:Gold@gold.example.com>;rc=1;index=1.1
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]

Barnes, et al. Informational [Page 22] RFC 7131 History-Info Call Flows March 2014

 F3 302 Moved Temporarily Gold.example.com -> example.com
 SIP/2.0 302 Moved Temporarily
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bK12s4;\
                received=192.0.2.101
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK42t2
 From: Alice <sip:alice@example.com>;tag=1235
 To: Gold Member Assistance <sip:Gold@example.com>;tag=kkaz-
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 History-Info: <sip:Gold@example.com>;index=1
 History-Info: <sip:Gold@gold.example.com>;rc=1;index=1.1
 Contact: <sip:Silver@example.com>;mp=1
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]
 F4 INVITE example.com -> Silver.example.com
 INVITE sip:Silver@example.com SIP/2.0
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bK45q2
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK42t2
 Max-Forwards: 69
 From: Alice <sip:alice@example.com>;tag=1235
 To: Gold Member Assistance <sip:Gold@example.com>
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 History-Info: <sip:Gold@example.com>;index=1
 History-Info: <sip:Gold@gold.example.com?Reason=SIP%3Bcause%3D302>;\
                 rc=1;index=1.1
 History-Info: <sip:Silver@example.com>;index=1.2;mp=1
 History-Info: <sip:Silver@silver.example.com>;index=1.2.1;rc=1.2
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]

Barnes, et al. Informational [Page 23] RFC 7131 History-Info Call Flows March 2014

 F5 INVITE Silver.example.com -> Agent
 INVITE sip:Silver@192.0.2.7 SIP/2.0
 Via: SIP/2.0/TCP silver.example.com:5060;branch=z9hG4bKerxs
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bK45q2;\
                received=192.0.2.101
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK42t2
 Max-Forwards: 68
 From: Alice <sip:alice@example.com>;tag=1235
 To: Gold Member Assistance <sip:Gold@example.com>
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 History-Info: <sip:Gold@example.com>;index=1
 History-Info: <sip:Gold@gold.example.com?Reason=SIP%3Bcause%3D302>;\
                 rc=1;index=1.1
 History-Info: <sip:Silver@example.com>;index=1.2;mp=1
 History-Info: <sip:Silver@silver.example.com>;index=1.2.1;rc=1.2
 History-Info: <sip:Silver@192.0.2.7>;index=1.2.1.1;rc=1.2.1
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]

Barnes, et al. Informational [Page 24] RFC 7131 History-Info Call Flows March 2014

 F6 200 OK Agent -> Silver.example.com
 SIP/2.0 200 OK
 Via: SIP/2.0/TCP silver.example.com:5060;branch=z9hG4bKerxs;\
                 received=192.0.2.5
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bK45q2;\
                 received=192.0.2.101
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK42t2
 From: Alice <sip:alice@example.com>;tag=1235
 To: Gold Member Assistance <sip:Gold@example.com>;tag=2325
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 History-Info: <sip:Gold@example.com>;index=1
 History-Info: <sip:Gold@gold.example.com?Reason=SIP%3Bcause%3D302>;\
                 rc=1;index=1.1
 History-Info: <sip:Silver@example.com>;index=1.2;mp=1
 History-Info: <sip:Silver@silver.example.com>;index=1.2.1;rc=1.2
 History-Info: <sip:Silver@192.0.2.7>;index=1.2.1.1;rc=1.2.1
 Contact: Agent <sip:Silver@192.0.2.7>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]
 F7 200 OK Silver.example.com -> example.com
 SIP/2.0 200 OK
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bK45q2;\
                 received=192.0.2.101
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK42t2
 From: Alice <sip:alice@example.com>;tag=1235
 To: Gold Member Assistance <sip:Gold@example.com>;tag=2325
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 History-Info: <sip:Gold@example.com>;index=1
 History-Info: <sip:Gold@gold.example.com?Reason=SIP%3Bcause%3D302>;\
                 rc=1;index=1.1
 History-Info: <sip:Silver@example.com>;index=1.2;mp=1
 History-Info: <sip:Silver@silver.example.com>;index=1.2.1;rc=1.2
 History-Info: <sip:Silver@192.0.2.7>;index=1.2.1.1;rc=1.2.1
 Contact: Agent <sip:Silver@192.0.2.7>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]

Barnes, et al. Informational [Page 25] RFC 7131 History-Info Call Flows March 2014

 F8 200 OK example.com -> Alice
 SIP/2.0 200 OK
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK42t2
 From: Alice <sip:alice@example.com>;tag=1235
 To: Gold Member Assistance <sip:Gold@example.com>;tag=2325
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 History-Info: <sip:Gold@example.com>;index=1
 History-Info: <sip:Gold@gold.example.com?Reason=SIP%3Bcause%3D302>;\
                 rc=1;index=1.1
 History-Info: <sip:Silver@example.com>;index=1.2;mp=1
 History-Info: <sip:Silver@silver.example.com>;index=1.2.1;rc=1.2
 History-Info: <sip:Silver@192.0.2.7>;index=1.2.1.1;rc=1.2.1
 Contact: Agent <sip:Silver@192.0.2.7>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]
 F9 ACK Alice -> Agent
 ACK sip:Silver@192.0.2.7 SIP/2.0
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK42t3
 Max-Forwards: 70
 From: Alice <sip:alice@example.com>;tag=1235
 To: Gold Member Assistance <sip:Gold@example.com>;tag=2325
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 ACK
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]
 The first hi-entry with the "mp" header field parameter contains an
 "mp" header field parameter value of 1, which points to the original-
 target, which allows the operator to identify that the call was from
 the Gold customer.

Barnes, et al. Informational [Page 26] RFC 7131 History-Info Call Flows March 2014

3.5. Determining the Alias Used

 SIP UAs are associated with an AOR.  It is possible for a single UA
 to actually have multiple AORs associated with it.  One common usage
 for this is aliases.  For example, a user might have an AOR of
 sip:john@example.com but also have the AORs
 sip:john.smith@example.com and sip:jsmith@example.com.  Rather than
 registering against each of these AORs individually, the user would
 register against just one of them, and the home proxy would
 automatically accept incoming calls for any of the aliases, treating
 them identically and ultimately forwarding them towards the UA.  This
 is common practice in the IP Multimedia Subsystem (IMS), where it is
 called "implicit registration" and each alias is called a "public
 user identity (PUID)".
 It is a common requirement for a User Agent Server (UAS), on receipt
 of a call, to know which of its aliases was used to reach it.  This
 knowledge can be used to choose ringtones to play, determine call
 treatment, and so on.  For example, a user might give out one alias
 to friends and family only, resulting in a special ring that alerts
 the user to the importance of the call.
 The following call flow and example messages show how History-Info
 can be used to find out the alias used to reach the callee.  The
 alias for the call is determined by hi-entry with the index that
 matches the value of the last hi-entry with an "rc" header field
 parameter in the Request received.
        Alice             example.com             John
        |                     | REGISTER F1         |
        |                     |<--------------------|
        |                     | 200 OK F2           |
        |                     |-------------------->|
        | INVITE F3           |                     |
        |-------------------->|                     |
        |                     | INVITE F4           |
        |                     |-------------------->|
                     * Rest of flow not shown *
                      Figure 5: Alias Example

Barnes, et al. Informational [Page 27] RFC 7131 History-Info Call Flows March 2014

 Message Details
 F1 REGISTER John -> example.com
 REGISTER sip:example.com SIP/2.0
 Via: SIP/2.0/TCP 192.0.2.1;branch=z9hG4bKnashds7
 Max-Forwards: 70
 From: John <sip:john@example.com>;tag=a73kszlfl
 To: John <sip:john@example.com>
 Supported: histinfo
 Call-ID: 1j9FpLxk3uxtm8tn@192.0.2.1
 CSeq: 1 REGISTER
 Contact: <sip:john@192.0.2.1>
 Content-Length: 0
 F2 200 OK example.com -> John
 SIP/2.0 200 OK
 Via: SIP/2.0/TCP 192.0.2.1;branch=z9hG4bKnashds7
 From: John <sip:john@example.com>;tag=a73kszlfl
 To: John <sip:john@example.com>;tag=d2dstee2
 Call-ID: 1j9FpLxk3uxtm8tn@192.0.2.1
 CSeq: 1 REGISTER
 Contact: <sip:john@192.0.2.1>;expires=3600
 Content-Length: 0
 F3 INVITE Alice -> example.com
 INVITE sip:john.smith@example.com SIP/2.0
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK42t2
 Max-Forwards: 70
 From: Alice <sip:alice@example.com>;tag=a73kszlfl
 To: John <sip:john.smith@example.com>
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 History-Info: <sip:john.smith@example.com>;index=1
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]

Barnes, et al. Informational [Page 28] RFC 7131 History-Info Call Flows March 2014

 F4 INVITE example.com -> John
 INVITE sip:john@192.0.2.1 SIP/2.0
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bK12s4
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK42t2
 Max-Forwards: 69
 From: Alice <sip:alice@example.com>;tag=a73kszlfl
 To: John <sip:john.smith@example.com>
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 Record-Route: <sip:proxy.example.com;lr>
 History-Info: <sip:john.smith@example.com>;index=1
 History-Info: <sip:john@192.0.2.1>;index=1.1;rc=1
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]
 The last hi-entry with the "rc" header field parameter references the
 source of retargeting pointing at the alias AOR, which in the example
 is "john.smith@example.com".

3.6. PBX Voicemail Example

 A typical use case for voicemail is one whereby the original called
 party is not reachable and the call arrives at a voicemail system.
 In some cases, multiple alternate destinations may be tried without
 success.  The voicemail system typically requires the original called
 party information to determine the appropriate mailbox so an
 appropriate greeting can be provided and the appropriate party
 notified of the message.
 In this example, Alice calls Bob, whose SIP client is forwarded to
 Carol.  Carol does not answer the call; thus, it is forwarded to a VM
 (voicemail) server (VMS).  In order to determine the appropriate
 mailbox to use for this call, the VMS needs the original target for
 the request.  The original target is determined by finding the first
 hi-entry tagged with "rc" or "mp" and using the hi-entry referenced
 by the index of "rc" or "mp" header field parameter as the target for
 determining the appropriate mailbox.  This hi-entry is used to
 populate the "target" URI parameter as defined in [RFC4458].  The
 reason associated with the first hi-entry tagged with "rc" or "mp"
 (i.e., 302) could be used to provide a customized voicemail greeting
 and is used to populate the "cause" URI parameter as defined in
 [RFC4458].  Note that some VMSs may also (or instead) use the

Barnes, et al. Informational [Page 29] RFC 7131 History-Info Call Flows March 2014

 information available in the History-Info headers for custom handling
 of the VM based on how and why the call arrived at the VMS.
 Furthermore, it is the proxy forwarding the call to the VMS that
 determines the target of the voicemail; it is the proxy that sets the
 target of voicemail, which is also the entity that utilizes [RFC7044]
 to find the target that is usually based on local policy installed by
 the user or an administrator.
 Alice      example.com       Bob          Carol        VM
 | INVITE F1    |              |             |          |
 |------------->|              |             |          |
 |              | INVITE  F2   |             |          |
 |              |------------->|             |          |
 |              |              |             |          |
 |  100 Trying  |              |             |          |
 |<-------------| 302 Moved Temporarily F3   |          |
 |              |<-------------|             |          |
 |              |              |             |          |
 |              |      ACK     |             |          |
 |              |------------->|             |          |
 |              |              |             |          |
 |              | INVITE F4    |             |          |
 |              |--------------------------->|          |
 |              |              |             |          |
 |              |         180 Ringing  F5    |          |
 |              |<---------------------------|          |
 |              |              |             |          |
 | 180 Ringing  |              |             |          |
 |<-------------|              |             |          |
 |              |              |             |          |
 |              |       (timeout)            |          |
 |              |              |             |          |
 |              | INVITE  F6   |             |          |
 |              |-------------------------------------->|
 |              |              |             |          |
 |              |               200 OK  F7              |
 |              |<--------------------------------------|
 |   200 OK     |              |             |          |
 |<-------------|              |             |          |
 |              |              |             |          |
 |                         ACK                          |
 |----------------------------------------------------->|
               Figure 6: Enterprise Voicemail Example

Barnes, et al. Informational [Page 30] RFC 7131 History-Info Call Flows March 2014

 Message Details
 F1 INVITE Alice -> example.com
 INVITE sip:bob@example.com SIP/2.0
 Via: SIP/2.0/TCP  192.0.2.3:5060;branch=z9hG4bK42t2
 Max-Forwards: 70
 From: Alice <sip:alice@example.com>;tag=kkaz-
 To: Bob <sip:bob@example.com>
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 History-Info: <sip:bob@example.com>;index=1
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Length: <appropriate value>
 [SDP Not Shown]
 F2 INVITE example.com -> Bob
 INVITE sip:bob@192.0.2.5 SIP/2.0
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bK12s4
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK42t2
 Max-Forwards: 69
 From: Alice <sip:alice@example.com>;tag=kkaz-
 To: Bob <sip:bob@example.com>
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 History-Info: <sip:bob@example.com>;index=1
 History-Info: <sip:bob@192.0.2.5>;index=1.1;rc=1
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]

Barnes, et al. Informational [Page 31] RFC 7131 History-Info Call Flows March 2014

 F3 302 Moved Temporarily Bob -> example.com
 SIP/2.0 302 Moved Temporarily
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bK12s4;\
                  received=192.0.2.101
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK42t2
 From: Alice <sip:alice@example.com>;tag=kkaz-
 To: Bob <sip:bob@example.com>;tag=2g22d-lnf
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 History-Info: <sip:bob@example.com>;index=1
 History-Info: <sip:bob@192.0.2.5>;index=1.1;rc=1
 Contact: <sip:carol@example.com>;mp=1
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]
 F4 INVITE example.com -> Carol
 INVITE sip:carol@192.0.2.4 SIP/2.0
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bK4522
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK42t2
 Max-Forwards: 69
 From: Alice <sip:alice@example.com>;tag=kkaz-
 To: Bob <sip:bob@example.com>
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 History-Info: <sip:bob@example.com>;index=1
 History-Info: <sip:bob@192.0.2.5?Reason=SIP%3Bcause%3D302>;\
                    index=1.1;rc=1
 History-Info: <sip:carol@example.com;cause=480>;index=1.2;mp=1
 History-Info: <sip:carol@192.0.2.4;cause=480>;index=1.2.1;rc=1.2
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]

Barnes, et al. Informational [Page 32] RFC 7131 History-Info Call Flows March 2014

 F5 180 Ringing Carol -> example.com
 SIP/2.0 180 Ringing
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bK4522;\
                  received=192.0.2.101
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK42t2
 From: Alice <sip:alice@example.com>;tag=kkaz-
 To: Bob <sip:bob@example.com>;tag=setss3x
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 History-Info: <sip:bob@example.com>;index=1
 History-Info: <sip:bob@192.0.2.5?Reason=SIP%3Bcause%3D302>;\
                    index=1.1;rc=1
 History-Info: <sip:carol@example.com;cause=480>;index=1.2;mp=1
 History-Info: <sip:carol@192.0.2.4;cause=480>;index=1.2.1;rc=1.2
 Contact: <sip:carol@192.0.2.4>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]

Barnes, et al. Informational [Page 33] RFC 7131 History-Info Call Flows March 2014

 F6 INVITE example.com -> VM
 INVITE sip:vm@192.0.2.6;target=sip:bob%40example.com;cause=480\
                      SIP/2.0
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bK4523
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK42t2
 Max-Forwards: 69
 From: Alice <sip:alice@example.com>;tag=kkaz-
 To: Bob <sip:bob@example.com>
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 History-Info: <sip:bob@example.com>;index=1
 History-Info: <sip:bob@192.0.2.5?Reason=SIP%3Bcause%3D302>;\
                     index=1.1;rc=1
 History-Info: <sip:carol@example.com;cause=480?Reason=SIP%3Bcause%3D\
               408>;index=1.2;mp=1
 History-Info: <sip:carol@192.0.2.4;cause=480?Reason=SIP%3Bcause%3D\
               408>;index=1.2.1;rc=1.2
 History-Info: <sip:vm@example.com;\
                     target=sip:bob%40example.com;cause=480>;\
                     index=1.3;mp=1
 History-Info: <sip:vm@192.0.2.6;\
                     target=sip:bob%40example.com;cause=480>;\
                     index=1.3.1;rc=1.3
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]

Barnes, et al. Informational [Page 34] RFC 7131 History-Info Call Flows March 2014

 F7 200 OK VM -> example.com
 SIP/2.0 200 OK
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bK4523;\
                  received=192.0.2.101
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK42t2
 From: Alice <sip:alice@example.com>;tag=kkaz-
 To: Bob <sip:bob@example.com>;tag=3dweggs
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 History-Info: <sip:bob@example.com>;index=1
 History-Info: <sip:bob@192.0.2.5?Reason=SIP%3Bcause%3D302>;\
                    index=1.1;rc=1
 History-Info: <sip:carol@example.com;cause=480?Reason=SIP%3Bcause%3D\
               408>;index=1.2;mp=1
 History-Info: <sip:carol@192.0.2.4;cause=480?Reason=SIP%3Bcause%3D\
               408>;index=1.2.1;rc=1.2
 History-Info: <sip:vm@example.com;\
                    target=sip:bob%40example.com;cause=480>;\
                    index=1.3;mp=1
 History-Info: <sip:vm@192.0.2.6;\
                    target=sip:bob%40example.com;cause=480>;\
                    index=1.3.1;rc=1.3
 Contact: <sip:vm@192.0.2.6>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]
 The VMS can look at the last hi-entry and find the target of the
 mailbox by looking at the URI entry in the "target" URI parameter in
 the hi-entry.

3.7. Consumer Voicemail Example

 In the case of a consumer, when the call is retargeted, it is usually
 to another administrative domain.  The voicemail system in these
 environments typically requires the last-called-party information to
 determine the appropriate mailbox so an appropriate greeting can be
 provided and the appropriate party notified of the message.
 In this example, Alice calls Bob, but Bob has temporarily forwarded
 his phone to Carol (she is his wife).  Carol does not answer the
 call; thus, it is forwarded to a VMS.  In order to determine the
 appropriate mailbox to use for this call, the VMS needs the
 appropriate target for the request.  The last target is determined by
 finding the hi-entry referenced by the index of last hi-entry tagged

Barnes, et al. Informational [Page 35] RFC 7131 History-Info Call Flows March 2014

 with "mp" for determining the appropriate mailbox.  This hi-entry is
 used to populate the "target" URI parameter as defined in [RFC4458].
 Note that some VMSs may also (or instead) use the information
 available in the History-Info headers for custom handling of the VM
 in terms of how and why the called arrived at the VMS.
 Alice      example.com       Bob          Carol        VM
 | INVITE F1    |              |             |          |
 |------------->|              |             |          |
 |              | INVITE  F2   |             |          |
 |              |------------->|             |          |
 |              |              |             |          |
 |  100 Trying  |              |             |          |
 |<-------------| 302 Moved Temporarily F3   |          |
 |              |<-------------|             |          |
 |              |              |             |          |
 |              |      ACK     |             |          |
 |              |------------->|             |          |
 |              |              |             |          |
 |              | INVITE F4    |             |          |
 |              |--------------------------->|          |
 |              |              |             |          |
 |              |         180 Ringing  F5    |          |
 |              |<---------------------------|          |
 |              |              |             |          |
 | 180 Ringing  |              |             |          |
 |<-------------|              |             |          |
 |              |              |             |          |
 |              |       (timeout)            |          |
 |              |              |             |          |
 |              | INVITE  F6   |             |          |
 |              |-------------------------------------->|
 |              |              |             |          |
 |              |               200 OK  F7              |
 |              |<--------------------------------------|
 |   200 OK     |              |             |          |
 |<-------------|              |             |          |
 |              |              |             |          |
 |                         ACK                          |
 |----------------------------------------------------->|
                Figure 7: Consumer Voicemail Example

Barnes, et al. Informational [Page 36] RFC 7131 History-Info Call Flows March 2014

 Message Details
 F1 INVITE Alice -> example.com
 INVITE sip:bob@example.com SIP/2.0
 Via: SIP/2.0/TCP  192.0.2.3:5060;branch=z9hG4bK42t2
 Max-Forwards: 70
 From: Alice <sip:alice@example.com>;tag=kkaz-
 To: Bob <sip:bob@example.com>
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 History-Info: <sip:bob@example.com>;index=1
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Length: <appropriate value>
 [SDP Not Shown]
 F2 INVITE example.com -> Bob
 INVITE sip:bob@192.0.2.5 SIP/2.0
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bK12s4
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK42t2
 Max-Forwards: 69
 From: Alice <sip:alice@example.com>;tag=kkaz-
 To: Bob <sip:bob@example.com>
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 History-Info: <sip:bob@example.com>;index=1
 History-Info: <sip:bob@192.0.2.5>;index=1.1;rc=1
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]

Barnes, et al. Informational [Page 37] RFC 7131 History-Info Call Flows March 2014

 F3 302 Moved Temporarily Bob -> example.com
 SIP/2.0 302 Moved Temporarily
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bK12s4;\
         received=192.0.2.101
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK42t2
 From: Alice <sip:alice@example.com>;tag=kkaz-
 To: Bob <sip:bob@example.com>;tag=224ls3s-t
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 History-Info: <sip:bob@example.com>;index=1
 History-Info: <sip:bob@192.0.2.5>;index=1.1;rc=1
 Contact: <sip:carol@example.com>;mp=1
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]
 F4 INVITE example.com -> Carol
 INVITE sip:carol@192.0.2.4 SIP/2.0
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bK24s5
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK42t2
 Max-Forwards: 69
 From: Alice <sip:alice@example.com>;tag=kkaz-
 To: Bob <sip:bob@example.com>
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 History-Info: <sip:bob@example.com>;index=1
 History-Info: <sip:bob@192.0.2.5?Reason=SIP%3Bcause%3D302\
               %3Btext%3D%22Moved%20Temporarily%22>\
               ;index=1.1;rc=1
 History-Info: <sip:carol@example.com>;index=1.2;mp=1
 History-Info: <sip:carol@192.0.2.4>;index=1.2.1;rc=1.2
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]

Barnes, et al. Informational [Page 38] RFC 7131 History-Info Call Flows March 2014

 F5 180 Ringing Carol -> example.com
 SIP/2.0 180 Ringing
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bK24s5;\
         received=192.0.2.101
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK42t2
 From: Alice <sip:alice@example.com>;tag=kkaz-
 To: Bob <sip:bob@example.com>;tag=setss3x
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 History-Info: <sip:bob@example.com>;index=1
 History-Info: <sip:bob@192.0.2.5?Reason=SIP%3Bcause%3D302\
               %3Btext%3D%22Moved%20Temporarily%22>;\
               index=1.1;rc=1
 History-Info: <sip:carol@example.com>;index=1.2;mp=1
 History-Info: <sip:carol@192.0.2.4>;index=1.2.1;rc=1.2
 Contact: <sip:carol@192.0.2.4>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]

Barnes, et al. Informational [Page 39] RFC 7131 History-Info Call Flows March 2014

 F6 INVITE example.com -> VM
 INVITE sip:vm@192.0.2.6;target=sip:carol%40example.com SIP/2.0
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bKbbg4
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK42t2
 Max-Forwards: 69
 From: Alice <sip:alice@example.com>;tag=kkaz-
 To: Bob <sip:bob@example.com>
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 History-Info: <sip:bob@example.com>;index=1
 History-Info: <sip:bob@192.0.2.5?Reason=SIP%3Bcause%3D302\
               %3Btext%3D%22Moved%20Temporarily%22>;\
               index=1.1;rc=1
 History-Info: <sip:carol@example.com>;\
               index=1.2;mp=1
 History-Info: <sip:carol@192.0.2.4?Reason=SIP%3Bcause%3D408>;\
               index=1.2.1;rc=1.2
 History-Info: <sip:vm@example.com;target=sip:carol%40example.com;\
               cause=408>;index=1.2.2;mp=1.2
 History-Info: <sip:vm@192.0.2.5;target=sip:carol%40example.com;\
               cause=408>;index=1.2.2.1;rc=1.2.2
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]

Barnes, et al. Informational [Page 40] RFC 7131 History-Info Call Flows March 2014

 F7 200 OK VM -> example.com
 SIP/2.0 200 OK
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bKbbg4
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK42t2
 From: Alice <sip:alice@example.com>;tag=kkaz-
 To: Bob <sip:bob@example.com>;tag=3dweggs
 Supported: histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 History-Info: <sip:bob@example.com>;index=1
 History-Info: <sip:bob@192.0.2.5?Reason=SIP%3Bcause%3D302\
               %3Btext%3D%22Moved%20Temporarily%22>;\
               index=1.1;rc=1
 History-Info: <sip:carol@example.com>;\
               index=1.2;mp=1
 History-Info: <sip:carol@192.0.2.4?Reason=SIP%3Bcause%3D408>;\
               index=1.2.1;rc=1.2
 History-Info: <sip:vm@example.com;target=sip:carol%40example.com;\
               cause=408>;index=1.2.2;mp=1.2
 History-Info: <sip:vm@192.0.2.5;target=sip:carol%40example.com;\
               cause=408>;index=1.2.2.1;rc=1.2.2
 Contact: <sip:carol@192.0.2.5>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]
 The VMS can look at the last hi-entry and find the target of the
 mailbox by looking for the "target" URI parameter in the hi-entry and
 the reason by the "cause" URI parameter in the same hi-entry.

3.8. GRUU

 A variation on the problem in Section 3.5 occurs with Globally
 Routable User Agent URI (GRUU) [RFC5627].  A GRUU is a URI assigned
 to a UA instance that has many of the same properties as the AOR but
 causes requests to be routed only to that specific instance.  It is
 desirable for a UA to know whether it was reached because a
 correspondent sent a request to its GRUU or to its AOR.  This can be
 used to drive differing authorization policies on whether the request
 should be accepted or rejected, for example.  However, like the AOR
 itself, the GRUU is lost in translation at the home proxy.  Thus, the
 UAS cannot know whether it was contacted via the GRUU or its AOR.

Barnes, et al. Informational [Page 41] RFC 7131 History-Info Call Flows March 2014

 The following call flow and example messages show how History-Info
 can be used to find out the GRUU used to reach the callee.
 While a GRUU is comprised of an AOR with a URI parameter, as defined
 in [RFC5627], the GRUU construct itself is not an AOR.  Thus, the
 retargeting of a request based on a GRUU does not result in the
 addition of an "rc" header field parameter to the hi-entry containing
 the GRUU.  The lack of an "rc" header field parameter in the hi-
 entries can be a hint that the source of retargeting is a GRUU.
 However, to ensure this is the case, the UAS needs to search for a
 "gr" parameter in the hi-entry prior to the last hi-entry.  If there
 is a GRUU, the URI will always be prior to the last hi-entry as the
 GRUU does not allow multiple instance to be mapped to a contact
 address.
        Alice             example.com             John
        |                     | REGISTER F1         |
        |                     |<--------------------|
        |                     | 200 OK F2           |
        |                     |-------------------->|
        | INVITE F3           |                     |
        |-------------------->|                     |
        |                     | INVITE F4           |
        |                     |-------------------->|
                     * Rest of flow not shown *
                       Figure 8: GRUU Example
 Message Details
 F1 REGISTER John -> example.com
 REGISTER sip:example.com SIP/2.0
 Via: SIP/2.0/UDP 192.0.2.1;branch=z9hG4bKnashds7
 Max-Forwards: 70
 From: John <sip:John@example.com>;tag=a73kszlfl
 Supported: gruu
 To: John <sip:john@example.com>
 Call-ID: 1j9FpLxk3uxtm8tn@192.0.2.1
 CSeq: 1 REGISTER
 Contact: <sip:john@192.0.2.1>;+sip.instance=\
     <urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>
 Content-Length: 0
 [SDP Not Shown]

Barnes, et al. Informational [Page 42] RFC 7131 History-Info Call Flows March 2014

 F2 200 OK example.com -> John
 SIP/2.0 200 OK
 Via: SIP/2.0/TCP 192.0.2.1;branch=z9hG4bKnashds7
 From: John <sip:john@example.com>;tag=a73kszlfl
 To: John <sip:john@example.com> ;tag=b88sn
 Call-ID: 1j9FpLxk3uxtm8tn@192.0.2.1
 CSeq: 1 REGISTER
 Contact: <sip:john@192.0.2.1>;\
     pub-gruu="sip:john@example.com;\
     gr=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6";\
     temp-gruu=\
     "sip:tgruu.7hs==jd7vnzga5w7fajsc7-ajd6fabz0f8g5@example.com;\
   gr";+sip.instance=\
     "<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>";\
     expires=3600
   Content-Length: 0
 [SDP Not Shown]
 Assuming Alice has knowledge of a GRUU either through
 prior communication or through other means such as presence
 places a call to John's GRUU.
 F3 INVITE Alice -> example.com
 INVITE sip:john@example.com;\
     gr=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6 SIP/2.0
 Via: SIP/2.0/TCP  192.0.2.3:5060;branch=z9hG4bK42t2
 Max-Forwards: 70
 From: Alice <sip:alice@example.com>;tag=kkaz-
 To: <sip:john@example.com;\
     gr=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>
 Supported: gruu, histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 History-Info: <sip:john@example.com;\
     gr=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>;index=1
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Length: <appropriate value>
 [SDP Not Shown]

Barnes, et al. Informational [Page 43] RFC 7131 History-Info Call Flows March 2014

 F4 INVITE example.com -> John
 INVITE sip:john@192.0.2.1 SIP/2.0
 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bK12s4
 Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK42t2
 Max-Forwards: 69
 From: Alice <sip:alice@example.com>;tag=kkaz-
 To: <sip:john@example.com;\
     gr=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>
 Supported: gruu, histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 Record-Route: <sip:proxy.example.com;lr>
 History-Info: <sip:john@example.com;\
     gr=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>;index=1
 History-Info: <sip:john@192.0.2.1>;index=1.1;rc=1
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]
 By analyzing the entry referenced by the entry with the last "rc",
 one can realize that the URI used to reach the device was GRUU by
 finding the "gr" parameter.

3.9. Limited-Use Address

 A limited-use address is a SIP URI that is minted on-demand, and
 passed out to a small number (usually one) of remote correspondents.
 Incoming calls targeted to that limited-use address are accepted as
 long as the UA still desires communications from the remote target.
 Should they no longer wish to be bothered by that remote
 correspondent, the URI is invalidated so that future requests
 targeted to it are rejected.
 Limited-use addresses are used in battling voice spam [RFC5039].  The
 easiest way to provide them would be for a UA to be able to take its
 AOR and "mint" a limited-use address by appending additional
 parameters to the URI.  It could then give out the URI to a
 particular correspondent and remember that URI locally.  When an
 incoming call arrives, the UAS would examine the parameter in the URI
 and determine whether or not the call should be accepted.
 Alternatively, the UA could push authorization rules into the
 network, so that it need not even see incoming requests that are to
 be rejected.

Barnes, et al. Informational [Page 44] RFC 7131 History-Info Call Flows March 2014

 This approach, especially when executed on the UA, requires that
 parameters attached to the AOR, but not used by the home proxy in
 processing the request, survive the translation at the home proxy and
 be presented to the UA.  This will not be the case with the logic in
 RFC 3261, since the Request-URI is replaced by the registered
 contact, and any such parameters are lost.
 Using the History-Info, John's UA can easily see if the call was
 addressed to its AOR, GRUU, or a temp-GRUU and treat the call
 accordingly by looking for a "gr" tag in the hi-entry prior to the
 last hi-entry.
        Alice             example.com             John
        |                     | REGISTER F1         |
        |                     |<--------------------|
        |                     | 200 OK F2           |
        |                     |-------------------->|
        | INVITE F3           |                     |
        |-------------------->|                     |
        |                     | INVITE F4           |
        |                     |-------------------->|
                     * Rest of flow not shown *
               Figure 9: Limited-Use Address Example
 Message Details
 F1 REGISTER John -> example.com
 REGISTER sip:example.com SIP/2.0
 Via: SIP/2.0/UDP 192.0.2.1;branch=z9hG4bKnashds7
 Max-Forwards: 70
 From: John <sip:John@example.com>;tag=a73kszlfl
 Supported: gruu
 To: John <sip:john@example.com>
 Call-ID: 1j9FpLxk3uxtm8tn@192.0.2.1
 CSeq: 1 REGISTER
 Contact: <sip:john@192.0.2.1>;\
   +sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"
 Content-Length: 0

Barnes, et al. Informational [Page 45] RFC 7131 History-Info Call Flows March 2014

 F2 200 OK example.com -> John
 SIP/2.0 200 OK
 Via: SIP/2.0/UDP 192.0.2.1;branch=z9hG4bKnashds7
 From: John <sip:john@example.com>;tag=a73kszlfl
 To: John <sip:john@example.com> ;tag=b88sn
 Call-ID: 1j9FpLxk3uxtm8tn@192.0.2.1
 CSeq: 1 REGISTER
 Contact: <sip:john@192.0.2.1>;\
   pub-gruu="sip:john@example.com;\
   gr=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6";\
   temp-gruu=\
   "sip:tgruu.7hs==jd7vnzga5w7fajsc7-ajd6fabz0f8g5@example.com;gr";\
   +sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>";\
   expires=3600
 Content-Length: 0
  Assuming Alice has knowledge of a temp-GRUU, she places a
  call to the temp-GRUU.
 F3 INVITE Alice -> example.com
 INVITE sip:tgruu.7hs==jd7vnzga5w7fajsc7-ajd6fabz0f8g5@example.com;\
     gr SIP/2.0
 Via: SIP/2.0/UDP 192.0.2.3:5060;branch=z9hG4bK42t2
 Max-Forwards: 70
 From: Alice <sip:alice@example.com>;tag=kkaz-
 To: <sip:tgruu.7hs==jd7vnzga5w7fajsc7-ajd6fabz0f8g5@example.com\
  ;gr>
 Supported: gruu, histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 History-Info: \
  <sip:tgruu.7hs==jd7vnzga5w7fajsc7-ajd6fabz0f8g5@example.com;gr>\
  ;index=1
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Length: <appropriate value>

Barnes, et al. Informational [Page 46] RFC 7131 History-Info Call Flows March 2014

 F4 INVITE example.com -> John
 INVITE sip:john@192.0.2.1 SIP/2.0
 Via: SIP/2.0/UDP proxy.example.com:5060;branch=z9hG4bK12s4
 Via: SIP/2.0/UDP 192.0.2.3:5060;branch=z9hG4bK42t2
 Max-Forwards: 69
 From: Alice <sip:alice@example.com>;tag=kkaz-
 To: <sip:tgruu.7hs==jd7vnzga5w7fajsc7-ajd6fabz0f8g5@example.com\
  ;gr>
 Supported: gruu, histinfo
 Call-ID: 12345600@example.com
 CSeq: 1 INVITE
 Record-Route: <sip:proxy.example.com;lr>
 History-Info: \
  <sip:tgruu.7hs==jd7vnzga5w7fajsc7-ajd6fabz0f8g5@example.com;gr>\
  ;index=1
 History-Info: <sip:john@192.0.2.1>;index=1.1;rc=1
 Contact: Alice <sip:alice@192.0.2.3>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 By analyzing the entry referenced by the entry with the last "rc",
 one can realize that the URI used to reach the device was GRUU by
 finding the "gr" parameter.

3.10. Service Invocation

 Several SIP specifications have been developed that make use of
 complex URIs to address services within the network rather than
 subscribers.  The URIs are complex because they contain numerous
 parameters that control the behavior of the service.  Examples of
 this include the specification that first introduced the concept,
 [RFC3087], control of network announcements and Interactive Voice
 Response (IVR) with SIP URI [RFC4240], and control of voicemail
 access with SIP URI [RFC4458].
 A common problem with all of these mechanisms is that once a proxy
 has decided to rewrite the Request-URI to point to the service, it
 cannot be sure that the Request-URI will not be destroyed by a
 downstream proxy that decides to forward the request in some way, and
 does so by rewriting the Request-URI.
 Section 3.6 shows how History-Info can be used to invoke a service.

Barnes, et al. Informational [Page 47] RFC 7131 History-Info Call Flows March 2014

3.11. Toll-Free Number

 Toll-free numbers, also known in the United States as 800 or 8xx
 numbers, are telephone numbers that are free for users to call.
 In the telephone network, toll-free numbers are just aliases to
 actual numbers that are used for routing of the call.  In order to
 process the call in the PSTN, a switch will perform a query (using a
 protocol called Transaction Capabilities Application Part (TCAP)),
 which will return either a phone number or the identity of a carrier
 which can handle the call.
 There has been recent work on allowing such PSTN translation services
 to be accessed by SIP proxy servers through IP querying mechanisms.
 For example, ENUM [RFC6117] has already been proposed as a mechanism
 for performing Number Portability (NP) queries [RFC4769].  Using it
 for 8xx number translations is a logical next step.
 The new target from translating the 8xx number may be in the PSTN or
 in the SIP network.  If the new target is an entity in the PSTN, the
 proper treatment in the PSTN (and in particular, correct
 reconciliation of billing records) requires that the call be marked
 with both the originating number (8xx number) and the new target
 number, History-info would come in play here to assure original 8xx
 number is not lost.
 Although not required to have both the originating number (8xx
 number) and the new target in the SIP network, an enterprise or user
 who utilize the 8xx service can benefit by knowing whether the call
 came in via an 8xx number in order to treat the call differently (for
 example, to play a special announcement), but if the original
 Request-URI is lost through translation, there is no way to tell if
 the call came in via 8xx number.  History-Info again would come in
 play here.
 Similar problems arise with other "special" numbers and services used
 in the PSTN, such as operator services, pay/premium numbers (9xx
 numbers in the United States), and short service codes such as 311.
 To find the service number, the UAS can extract the hi-entry whose
 index matches the value of the first hi-entry with an "mp" tag.
 Technically, the call can be forwarded to these "special" numbers
 from non-special numbers; however, that is uncommon based on the way
 these services authorize translations.
 This example call flow shows a UAC that does not support History-
 Info.

Barnes, et al. Informational [Page 48] RFC 7131 History-Info Call Flows March 2014

       Alice      Toll-Free Service   Atlanta.com          John
        |                |              |                   |
        |    INVITE F1   |              |                   |
        |--------------->|   INVITE F2  |                   |
        |                |------------->|                   |
        |                |              |  INVITE F3        |
        |                |              |------------------>|
  • Rest of flow not shown *
                 Figure 10: Service Number Example
 Message Details
 F1 INVITE 192.0.2.1 -> Toll-Free Service
 INVITE sip:+18005551002@example.com;user=phone  SIP/2.0
 Via: SIP/2.0/TCP 192.0.2.1:5060;branch=z9hG4bK74bf
 From: Alice <sip:+15551001@example.com;user=phone>;tag=9fxced76sl
 To: <sip:+18005551002@example.com;user=phone>
 Call-ID: c3x842276298220188511
 CSeq: 1 INVITE
 Max-Forwards: 70
 Contact: <sip:alice@192.0.2.1>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]

Barnes, et al. Informational [Page 49] RFC 7131 History-Info Call Flows March 2014

 F2 INVITE Toll-Free Service -> Atlanta.com
 INVITE sip:+15555551002@atlanta.com SIP/2.0
 Via: SIP/2.0/TCP 192.0.2.4:5060;branch=z9hG4bK-ik8
 Via: SIP/2.0/TCP 192.0.2.1:5060;branch=z9hG4bK74bf
 From: Alice <sip:+15551001@example.com;user=phone>;tag=9fxced76sl
 To: <sip:+18005551002@example.com;user=phone>
 Call-ID: c3x842276298220188511
 CSeq: 1 INVITE
 Max-Forwards: 69
 Supported: histinfo
 History-Info: <sip:+18005551002@example.com;user=phone>;index=1
 History-Info: <sip:+15555551002@atlanta.com>;index=1.1;mp=1
 Contact: <sip:alice@192.0.2.1>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]
 F3 INVITE Atlanta.com -> John
 INVITE sip:john@198.51.100.2 SIP/2.0
 Via: SIP/2.0/TCP 198.51.100.1:5060;branch=z9hG4bKpxk7g
 Via: SIP/2.0/TCP 192.0.2.4:5060;branch=z9hG4bK-ik8
 Via: SIP/2.0/TCP 192.0.2.1:5060;branch=z9hG4bK74bf
 From: Alice <sip:+15551001@example.com;user=phone>;tag=9fxced76sl
 To: <sip:+18005551002@example.com;user=phone>
 Call-ID: c3x842276298220188511
 CSeq: 1 INVITE
 Max-Forwards: 68
 Supported: histinfo
 History-Info: <sip:+18005551002@example.com;user=phone>;index=1
 History-Info: <sip:+15555551002@atlanta.com>;index=1.1;mp=1
 History-Info: <sip:john@atlanta.com>;index=1.1.1;rc=1.1
 History-Info: <sip:john@198.51.100.2>;index=1.1.1.1;rc=1.1.1
 Contact: <sip:alice@192.0.2.1>
 Content-Type: application/sdp
 Content-Length: <appropriate value>
 [SDP Not Shown]

Barnes, et al. Informational [Page 50] RFC 7131 History-Info Call Flows March 2014

4. Security Considerations

 The security considerations for the History-Info header field are
 specified in [RFC7044].

5. Acknowledgements

 Jonathan Rosenberg, et al produced the document that provided
 additional use cases precipitating the requirement for the new
 "target" parameter in the History-Info header field and the new SIP/
 SIPS URI parameter.  Hadriel Kaplan provided some comments.
 Brett Tate, Roland Jesske, Laura Liess, Scott Godin, Dale Worley, and
 Marianne Mohali provided extensive review and comments on call flows,
 message examples, and text.

6. Informative References

 [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
            A., Peterson, J., Sparks, R., Handley, M., and E.
            Schooler, "SIP: Session Initiation Protocol", RFC 3261,
            June 2002.
 [RFC5627]  Rosenberg, J., "Obtaining and Using Globally Routable User
            Agent URIs (GRUUs) in the Session Initiation Protocol
            (SIP)", RFC 5627, October 2009.
 [RFC3087]  Campbell, B. and R. Sparks, "Control of Service Context
            using SIP Request-URI", RFC 3087, April 2001.
 [RFC4240]  Burger, E., Van Dyke, J., and A. Spitzer, "Basic Network
            Media Services with SIP", RFC 4240, December 2005.
 [RFC5039]  Rosenberg, J. and C. Jennings, "The Session Initiation
            Protocol (SIP) and Spam", RFC 5039, January 2008.
 [RFC4458]  Jennings, C., Audet, F., and J. Elwell, "Session
            Initiation Protocol (SIP) URIs for Applications such as
            Voicemail and Interactive Voice Response (IVR)", RFC 4458,
            April 2006.
 [RFC6117]  Hoeneisen, B., Mayrhofer, A., and J. Livingood, "IANA
            Registration of Enumservices: Guide, Template, and IANA
            Considerations", RFC 6117, March 2011.

Barnes, et al. Informational [Page 51] RFC 7131 History-Info Call Flows March 2014

 [RFC4769]  Livingood, J. and R. Shockey, "IANA Registration for an
            Enumservice Containing Public Switched Telephone Network
            (PSTN) Signaling Information", RFC 4769, November 2006.
 [RFC7044]  Barnes, M., Audet, F., Schubert, S., van Elburg, J., and
            C. Holmberg, "An Extension to the Session Initiation
            Protocol (SIP) for Request History Information", RFC 7044,
            February 2014.

Authors' Addresses

 Mary Barnes
 TX
 US
 EMail: mary.ietf.barnes@gmail.com
 Francois Audet
 Skype
 EMail: francois.audet@skype.net
 Shida Schubert
 NTT
 Tokyo
 Japan
 EMail: shida@ntt-at.com
 Hans Erik van Elburg
 Detecon International Gmbh
 Oberkasseler str. 2
 Bonn
 Germany
 EMail: ietf.hanserik@gmail.com
 Christer Holmberg
 Ericsson
 Hirsalantie 11, Jorvas
 Finland
 EMail: christer.holmberg@ericsson.com

Barnes, et al. Informational [Page 52]

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